In a typical image forming method for either printing or transmitting an output based on image data having planes corresponding to plurality of color components, 1-bit raster data is converted into respective pieces of 8-bit RGB raster data. Then, image data having of planes of color components of RGB is generated. Subsequently, the output based on the image data is either printed or transmitted.
In the typical image forming method, as illustrated in FIG. 7, when a stamp image is combined with image data 91 formed of planes 91a, 91b, and 91c of a respective plurality of color components, color-component-specific stamp images 92a, 92b, and 92c obtained by decomposing the stamp image into color components corresponding to the planes 91a, 91b, and 91c of the image data 91 are generated. Then, in the typical image forming method, the color-component-specific stamp images 92a, 92b, and 92c are combined with the corresponding planes 91a, 91b, and 91c. 
In the typical image forming method, a number of color-component-specific stamp images equal to the number of planes of the image data are generated, which necessitates a large amount of memory capacity to store the color-component-specific stamp images.